Wireless LAN with central management of access points

ABSTRACT

A method for mobile communication includes arranging a plurality of access points in a wireless local area network (WLAN) to communicate over the air with a mobile station using a common basic service set identification (BSSID) for all the access points. Upon receiving at one or more of the access points an uplink signal transmitted over the WLAN by the mobile station using the common BSSID, messages are sent and received over a communication medium linking the access points in order to select one of the access points to respond to the uplink signal. The selected one of the access points transmits a response to the mobile station.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 10/214,271, filed Aug. 7, 2002, which is assignedto the assignee of the present patent application and whose disclosureis incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to wirelesscommunications, and specifically to methods and devices for improvingthe performance of wireless local area networks.

BACKGROUND OF THE INVENTION

[0003] Wireless local area networks (WLANs) are gaining in popularity,and new wireless applications are being developed. The original WLANstandards, such as “Bluetooth” and IEEE 802.11, were designed to enablecommunications at 1-2 Mbps in a band around 2.4 GHz. More recently, IEEEworking groups have defined the 802.11a, 802.11b and 802.11g extensionsto the original standard, in order to enable higher data rates. The802.11a standard, for example, envisions data rates up to 54 Mbps overshort distances in a 5 GHz band, while 802.11b defines data rates up to22 Mbps in the 2.4 GHz band. In the context of the present patentapplication and in the claims, the term “802.11” is used to refercollectively to the original IEEE 802.11 standard and all its variantsand extensions, unless specifically noted otherwise.

[0004] The theoretical capability of new WLAN technologies to offer highcommunication bandwidth to mobile users is severely hampered by thepractical limitations of wireless communications. Indoor propagation ofradio frequencies is not isotropic, because radio waves are influencedby building layout and furnishings. Therefore, even when wireless accesspoints are carefully positioned throughout a building, some “blackholes” generally remain—areas with little or no radio reception.Furthermore, 802.11 wireless links can operate at full speed only underconditions of high signal/noise ratio. Signal strength scales inverselywith the distance of the mobile station from its access point, andtherefore so does communication speed. A single mobile station with poorreception due to distance or radio propagation problems can slow downWLAN access for all other users in its basic service set (BSS−the groupof mobile stations communicating with the same access point in aconventional 802.11 WLAN).

[0005] The natural response to these practical difficulties would be todistribute a greater number of access points within the area to beserved. If a WLAN receiver receives signals simultaneously from twosources of similar strength on the same frequency channel, however, itis generally unable to decipher either signal. The 802.11 standardprovides a mechanism for collision avoidance known as clear channelassessment (CCA), which requires a station to refrain from transmittingwhen it senses other transmissions on its frequency channel. Inpractice, this mechanism is of limited utility and can place a heavyburden on different BSSs operating on the same frequency channel.

[0006] Therefore, in 802.11 WLANs known in the art, access points inmutual proximity must use different frequency channels. Theoretically,the 802.11b and 802.11g standards define 14 frequency channels in the2.4 GHz band, but because of bandwidth and regulatory limitations, WLANsoperating according to these standards in the United States actuallyhave only three different frequency channels from which to choose. (Inother countries, such as Spain, France and Japan, only one channel isavailable.) As a result, in complex, indoor environments, it becomespractically impossible to distribute wireless access points closelyenough to give strong signals throughout the environment withoutsubstantial overlap in the coverage areas of different access pointsoperating on the same frequency channel.

SUMMARY OF THE INVENTION

[0007] The present invention provides methods and devices for enhancingthe coverage and speed of WLAN systems. In embodiments of the presentinvention, a WLAN system comprises multiple wireless access pointsdistributed within a service region. The access points communicate withmobile stations in the service region in accordance with one or more ofthe 802.11 standards. In order to provide complete coverage of theservice region, with strong communication signals throughout the region,the access points may be closely spaced, and their areas of coverage maysubstantially overlap one another. The access points are linked by acommunication medium, such as a wired LAN, to an access point manager,which coordinates and controls the operations of the access points asdescribed hereinbelow.

[0008] In embodiments of the present invention, all the access pointsthat operate on a given frequency channel within a given service regionbelong to the same basic service set (BSS) and thus share the same BSSidentification (BSSID). (By contrast, in 802.11 WLAN systems known inthe art, each BSS includes only a single access point.) Therefore, inthese embodiments, any of the access points that are within range of agiven mobile station are capable of receiving and responding to uplinkmessages (also referred to as uplink signals) from the mobile station.These access points forward uplink management and broadcast messages tothe access point manager, which selects one of the access points torespond to the mobile station and to receive subsequent data messagesfrom the mobile station. The access point manager assigns one accesspoint to serve each mobile station in the WLAN. Problems of overlappingcoverage areas and collisions are thus resolved, typically in favor ofthe access point that has the strongest reception of uplink signals fromthe mobile station in question. The access points may therefore bedeployed within the service region as closely as desired, so that mobilestations everywhere in the service region experience good radio coverageand can operate at optimal speed.

[0009] In the above-mentioned parent application (U.S. Ser. No.10/214,271), multiple access points operating on the same frequencychannel similarly share the same BSSID. That application defines anarbitration procedure, whereby the access points communicate amongthemselves in order to select the access point that is to respond toeach uplink message. Embodiments of the present invention provide analternative solution, in which the access point manager selects theaccess point to respond to the mobile station. This solution reduces thecomputational demands on the access points that are associated withBSSID sharing, and may be implemented in pre-existing WLAN installationsand other WLANs using legacy access point hardware. This alternativesolution, however, introduces additional latency in the process ofselecting the access point that is to respond to an uplink message froma mobile station. Therefore, the access point is selected once at theinitiation of communications, and changes thereafter only occasionally,as described below.

[0010] In order to reduce latency in responding to uplink data messages,each access point is assigned its own address, typically a medium accesscontrol (MAC) address, in addition to the BSSID. Once a mobile stationhas connected to the WLAN and associated itself with the BSS, theselected access point transmits a downlink message to the mobilestation, instructing the mobile station to direct all uplink datamessages to the address of the selected access point. This downlinkmessage may be generated using a standard network control protocol, suchas the well-known Address Resolution Protocol (ARP), so that no specialprogramming of either the mobile station is required for this purpose.The access point to which the subsequent uplink messages are directedwill immediately acknowledge such messages, while other access points(with different network addresses) will ignore them. If the mobilestation changes position, or network conditions change, the access pointmanager may select a different access point to respond to subsequentcommunications with the mobile station. In this case, the mobile stationis “handed over” to the new access point by means of a downlink messageinstructing the mobile station to direct subsequent uplink data messagesto the address of the new access point.

[0011] Although the embodiments described herein make reference to the802.11 family of standards and use terminology taken from thesestandards, the principles of the present invention may similarly beapplied in wireless LANs of other types, based on other standards andcommunication protocols.

[0012] There is therefore provided, in accordance with an embodiment ofthe present invention, a method for mobile communication, including:

[0013] arranging a plurality of access points in a wireless local areanetwork (WLAN) to communicate over the air with a mobile station using acommon basic service set identification (BSSID) for all the accesspoints;

[0014] receiving at one or more of the access points an uplink signaltransmitted over the WLAN by the mobile station using the common BSSID;

[0015] sending and receiving messages over a communication mediumlinking the access points in order to select one of the access points torespond to the uplink signal; and

[0016] transmitting a response from the selected one of the accesspoints to the mobile station.

[0017] Typically, the access points are configured to communicate withthe mobile station over a common frequency channel shared by all theaccess points. The access points have respective service areas, and maybe arranged so that the service areas substantially overlap. Indisclosed embodiments, the access points are arranged to communicatewith the mobile station substantially in accordance with IEEE Standard802.11.

[0018] In disclosed embodiments, arranging the plurality of the accesspoints includes assigning a respective medium access control (MAC)address to each of the access points, so that each of the access pointsignores uplink data messages that are not addressed to the respectiveMAC address. Typically, arranging the plurality of the access pointsincludes configuring the access points to emulate mobile stationcommunications, so that each of the access points acknowledges theuplink data messages that are addressed from the mobile station to therespective MAC address. Sending and receiving the messages may includereconfiguring the selected one of the access points temporarily to stopemulating the mobile station communications, so as to transmit anacknowledgment to a management frame transmitted by the mobile station.Additionally or alternatively, sending and receiving the messages mayinclude changing the respective MAC address of the selected one of theaccess points temporarily, so as to cause the selected one of the accesspoints to transmit an acknowledgment to a management frame transmittedby the mobile station.

[0019] Typically, transmitting the response includes instructing themobile station to transmit all the uplink data messages to therespective MAC address of the selected one of the access points.Instructing the mobile station may include sending an Address ResolutionProtocol (ARP) response to the mobile station. In a disclosedembodiment, the method includes, subsequently to transmitting theresponse from the selected one of the access points, and responsively toa further uplink signal received from the mobile station, selecting afurther one of the access points to communicate with the mobile station,and sending a spoofed ARP response to the mobile station instructing themobile station to transmit all further uplink data messages to therespective MAC address of the further one of the access points.

[0020] Typically, receiving the uplink signal includes measuring astrength of the uplink signal at each of the one or more of the accesspoints, and sending and receiving the messages includes indicating inthe messages the measured strength of the uplink signal, and selectingthe one of the access points to respond to the uplink signalresponsively to the strength indicated in the messages. Optionally,measuring the strength includes measuring the strength repeatedly inresponse to subsequent uplink signals transmitted by the mobile station,and selecting the one of the access points includes selecting adifferent one of the access points to respond to the subsequent uplinksignals, responsively to a change in the measured strength.

[0021] There is also provided, in accordance with an embodiment of thepresent invention, a method for mobile communication, including:

[0022] arranging a plurality of access points in a wireless local areanetwork (WLAN) to communicate over the air on a common frequency channelwith a mobile station;

[0023] receiving at one or more of the access points an uplink signaltransmitted over the WLAN by the mobile station on the common frequencychannel;

[0024] conveying messages responsively to the uplink signal from the oneor more of the access points over a communication medium linking theaccess points to a manager node;

[0025] processing the messages at the manager node so as to select oneof the access points to respond to the uplink signal; and

[0026] transmitting a response from the selected one of the accesspoints to the mobile station.

[0027] In some embodiments, the manager node includes a plurality ofmanagement processors. Typically, the plurality of management processorsincludes a control processor and a packet processor, and processing themessages includes selecting the one of the access points to respond tothe uplink signal using the control processor, and further includesprocessing uplink data packets received by the selected one of theaccess points using the packet processor. Processing the uplink datapackets may include decrypting the uplink data packets and encryptingdownlink data packets at the packet processor, for transmission by theselected one of the access points. Additionally or alternatively,processing the messages includes distributing the messages forprocessing among the plurality of the management processors.

[0028] There is additionally provided, in accordance with an embodimentof the present invention, apparatus for mobile communication, including:

[0029] a plurality of access points, which are arranged in a wirelesslocal area network (WLAN) to communicate over the air on a commonfrequency channel with a mobile station using a common basic service setidentification (BSSID) for all the access points, and which are adapted,upon receiving at one or more of the access points an uplink signaltransmitted over the WLAN by the mobile station on the common frequencychannel, to convey messages responsively to the uplink signal from theone or more of the access points over a communication medium linking theaccess points; and

[0030] a manager node, linked to the communication medium, which isadapted to process the messages so as to select one of the access pointsto respond to the uplink signal, and to instruct the selected one of theaccess points to transmit a response to the mobile station.

[0031] The present invention will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a block diagram that schematically illustrates a WLANsystem, in accordance with a preferred embodiment of the presentinvention;

[0033]FIGS. 2A and 2B are a flow chart that schematically illustrates amethod for communication between a mobile station and access points in aWLAN system, in accordance with an embodiment of the present invention;

[0034]FIG. 3 is a flow chart that schematically illustrates a method forhanding over a mobile station from one serving access point to another,in accordance with an embodiment of the present invention; and

[0035]FIG. 4 is a flow chart that schematically illustrates a method forresponding to an uplink message from a mobile station, in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0036]FIG. 1 is a block diagram that schematically illustrates awireless LAN (WLAN) system 20, in accordance with a preferred embodimentof the present invention. System 20 comprises multiple access points 22,which are configured for data communication with mobile stations 24. Themobile stations typically comprise computing devices, such as desktop,portable or handheld devices. In the exemplary embodiments describedhereinbelow, it is assumed that the access points and mobile stationscommunicate with one another in accordance with one of the standards inthe IEEE 802.11 family and observe the 802.11 medium access control(MAC) layer conventions. Details of the 802.11 MAC layer are describedin ANSI/IEEE Standard 801.11 (1999 Edition), and specifically in Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)Specifications, which is incorporated herein by reference. Theprinciples of the present invention, however, are not limited to the802.11 standards, and may likewise be applied to substantially any typeof WLAN, including HiperLAN, Bluetooth and hiswan-based systems.

[0037] Access points 22 are connected by a communication medium,typically comprising a wired LAN 28 with a hub 26, typically an Ethernetswitching hub. LAN 28 serves as the distribution system (DS) forexchanging data between the access points and the hub. Hub 26 connectsthe access points to a node on LAN 28 that serves as an access pointmanager 30, which is also referred to hereinbelow as a BSS manager(BSSMGR). Manager 30 typically comprises a computer, operating under thecontrol of software suitable for carrying out the functions describedhereinbelow. The software may be downloaded to manager 30 in electronicform, or it may alternatively be provided on tangible media, such asCD-ROM. Alternatively or additionally, manager 30 may comprise dedicatedhardware circuits in order to accelerate the performance of itsfunctions. Typically, manager 30 is also linked to an external network32, such as the Internet, via an access line 34, so as to enable mobilestations 24 to send and receive data through access points 22 to andfrom the external network.

[0038] Although manager 30 is shown and described here as a single unit,in practice, the functions of the manager may be divided among two ormore units, which may occupy separate nodes in system 20. For example,manager 30 may be split into two logical entities: a control processor36 for handling management traffic (such as the association process,access point selection and handovers, as described hereinbelow), and apacket processor 38 for handling data traffic to and from access points22. The packet processor (whether integrated with or separate from thecontrol processor) may also perform packet encryption and decryptionfunctions, which are typically performed by the access points themselvesin WLAN systems known in the art. The control processor and packetprocessor may be contained physically in a single box or in separateboxes. If separate boxes are used, then at least the packet processor ispreferably connected to LAN 28, for rapid communication with the accesspoints. The control processor may be more remotely located, if desired,with an appropriate communication link between the packet and controlprocessors.

[0039] Furthermore, some or all of the functions of manager 30 may bedistributed among a plurality of manager nodes or packet processors.This distributed functionality has the potential advantages ofredundancy, making system 20 immune to a single point of failure, andreduced bandwidth requirements, by splitting packet traffic amongdifferent processors. When multiple packet processors 38 are used, forexample, each mobile station 24 may be assigned to one of the packetprocessors in order to balance the computational load among theprocessors. This assignment of packet processors 38 to mobile stations24 may be performed by control processor 36, which then informs accesspoints 22 of the packet processor that is assigned to serve each mobilestation.

[0040] Notwithstanding the possibilities of functional and physicalseparation of manager 30 into multiple separate units, in theembodiments described below manager 30 is treated as a single functionalunit for the sake of conceptual simplicity and clarity.

[0041] Access points 22 in system 20 are preferably closely spaced,operate on a common frequency channel, and share a common BSSID. Forincreased capacity, system 20 may include other, similar groups ofaccess points (not shown in the figure), operating on other frequencychannels, to the extent permitted by the applicable WLAN standard andregulatory regulations. Each of these groups has its own BSSID andoperates in a substantially identical manner to that describedhereinbelow with respect to access points 22. These other groups ofaccess points may be connected to the same LAN 28 and may be managed bythe same manager 30, or they may be managed independently by anothermanager. In any case, because they operate on different frequencychannels, the different groups operate substantially independently ofone another and may be regarded as separate systems. It is thereforesufficient to describe the operation of a single group, as shown in FIG.1, in order to provide an understanding of the present invention.

[0042] Since access points 22 operate on the same frequency channel,radio waves may typically reach mobile station 24 from multiple accesspoints simultaneously on this frequency channel. By the same token,uplink radio messages transmitted by mobile station 24 may be receivedat about the same time by multiple access points. In WLAN systems knownin the art, under these circumstances, mobile station 24 would receivedownlink acknowledgment and response messages from two or more of theaccess points, which would probably result in inability of the mobilestation to communicate with any of the access points. In embodiments ofthe present invention, manager 30 resolves this conflict by selectingone of access points 22 a priori to communicate with each mobile station(usually the closest access point to the mobile station, meaning theaccess point that received the uplink signal with the highest signalstrength). Manager 30 conveys response instructions to the selectedaccess point over LAN 28, as described hereinbelow. The other accesspoints meanwhile refrain from interfering.

[0043] Implementing this mode of operation requires modifying thebehavior of the access points, relative to 802.11 access points known inthe art. The 802.11 standard provides that an access point shouldacknowledge and respond to all uplink messages that are directed to theBSSID of the access point. Thus, if access points 22 were configured inthe conventional manner, all the access points would acknowledge theuplink messages from mobile stations 24, without waiting forinstructions from manager 30.

[0044] To overcome this limitation, while still using conventional WLANhardware, access points 22 are configured to emulate mobile stations andare assigned their own, unique MAC addresses. Some off-shelf WLANinterface chips, such as WLAN chipsets produce by Atheros Communications(Sunnyvale, Calif.), permit this reconfiguration to be carried out by anappropriate firmware command to the chip. Access points using thischipset, which may be adapted to operate in system 20, include the WLANAP 8500 or 8700 access points produced by 3COM (Santa Clara, Calif.) andthe Orinoco AP600a, AP-600 g and AP-2000 access points produced byProxim Corporation (Sunnyvale, Calif.). When configured to emulatemobile stations in this manner, each access point 22 acknowledges uplinkdata messages only when the data messages are directed to its assignedMAC address, and passes these messages over LAN 28 to manager 30. Theaccess point in this configuration ignores uplink data messages that aredirected to other MAC addresses, except to measure the signal strengthsof these uplink transmissions, as described below. The access pointstill receives and passes on control and management uplink messages overLAN 28 to manager 30, however, since these messages contain a specialMAC address, equal to the BSSID of the access point.

[0045] The use of conventional WLAN hardware to achieve the novelfunctionality of the present invention is generally advantageous inreducing the cost of system 20. Alternatively, access points 22 maycomprise custom or semi-custom hardware, designed for the purposes ofthe present invention. In any case, the novel operation of system 20 istransparent to mobile stations 24, which operate in accordance with the802.11 standard without modification.

[0046]FIGS. 2A and 2B are a flow chart that schematically illustratescommunication between mobile station 24 and access points 22 in system20, in accordance with an embodiment of the present invention. The flowchart is arranged to show the sequence of messages and associatedactions performed by the mobile station, by two access points(identified arbitrarily AP1 and AP2), and by manager 30 (BSSMGR). Forthe sake of simplicity, only these two access points are represented inthis figure, and AP1 is selected by manager 30 to respond to mobilestation 24. In practice, this method is typically carried out over alarger group of access points.

[0047] Communication between mobile station 24 and access points 22under the 802.11 standard begins with a probe request 40, in order toidentify available access points to which the mobile station mayconnect. Access points 22 that receive this request typically measurethe strength of the signal. The access points then forward the proberequest to manager 30, at a forwarding step 42. Optionally, the accesspoints forward a received signal strength indication (RSSI) togetherwith the probe request. Alternatively, RSSI measurements may be sentperiodically, and not for every packet received, in order to reducebandwidth consumption on LAN 28.

[0048] Manager 30 selects an access point (AP1 in the present example)to respond to mobile station 24, at an access point selection step 44.Typically, the access point reporting the strongest RSSI for a givenmobile station is in the best position to communicate with that mobilestation at the highest supportable data rate, and the manager thereforechooses this access point to respond to the mobile station.Alternatively, manager 30 may initially select at random the accesspoint that is to respond to probe request 40, and may subsequentlyappoint a new access point to communicate with the mobile station basedon the RSSI. As noted above, the access points typically measure thestrength of the signal for every uplink packet they receive, and thenperiodically transmit the updated RSSI measurements to manager 30, whichmay lead the manager to change the access point that is selected torespond to a given mobile station. Alternatively or additionally, themanager may take other factors into consideration, such as loadbalancing among the access points, in order to select the access pointto respond in each case. In any case, selection of the access point inthis manner, from among access points closely within the service regionof system 20, allows the mobile stations to communicate over the WLANsystem with generally better signal quality, and therefore higher datarates, than in WLAN systems known in the art.

[0049] Upon selecting access point AP1 to respond to the probe requestat step 44, manager 30 generates an appropriate response and conveys theresponse to the selected access point. AP1 returns a probe response 46,as instructed by manager 30, to mobile station 24. Other access pointsdo not respond. The mobile station then submits an authenticationrequest 48, in accordance with the 802.11 standard. The authenticationrequest specifies the BSSID which, as noted above, is shared by both AP1and AP2. Therefore, the authentication request is, again, received byboth AP1 and AP2, and forwarded to manager 30 at a forwarding step 50.

[0050] According to the 802.11 standard, after submitting request 48,the mobile station will expect to receive an acknowledgment (ACK) fromthe appropriate access point within a short period, typically 10 μs Ifthe acknowledgment is not received in time, the mobile station willretransmit the association request until it receives a response. If noresponse is received within a predetermined time period, the mobilestation will treat the association request as having failed. The cycleof forwarding association requests (and other messages) to manager 30and waiting for the manager to respond typically takes longer than the10 μs maximum set by the standard. An optional procedure for dealingwith this delay, in order to reduce the number of retransmissions andavoid failures due to timeout, is described below with reference to FIG.4.

[0051] In reply to authentication request 48, manager 30 generates theappropriate authentication response and passes response instructions tothe selected access point (AP1), at a response generation step 52. AP1then returns an authentication response 54 to mobile station 24,indicating that the mobile station is authorized to continuecommunications with the WLAN system.

[0052] The next step in the 802.11 communication management process isfor mobile station 24 to submit an association request 56, asking to beassociated with the BSS of access points 22. This request also specifiesthe BSSID shared by AP1 and AP2. The access points forward theassociation request to manager 30 for processing, at a forwarding step58. If necessary, AP1 may be prompted to return an acknowledgment, asnoted above and shown in FIG. 4. In reply to the association request,manager 30 generates the appropriate association response and passesresponse instructions to AP1, at a response generation step 60. AP1accordingly returns an association response 62 to mobile station 24. Themobile station is now ready to begin data communications with system 20.Optionally, data communications may be preceded by additionalmanagement-related communication steps, such as exchange of EAPOL(Extensible Authentication Protocol Over LAN) packets between mobilestation 24 and access points 22, as is known in the art. Theseadditional steps are handled in similar manner to the association stepsdescribed above.

[0053] As a precursor to application-level data communications, mobilestation 24 must typically first ascertain the network address of thedestination server or peer computer with which it wishes to communicate.The mobile station must also determine the MAC address of thedestination or of the router through which packets to this destinationshould be sent (if the destination is outside the subnet in which themobile station is located). Various protocols are known in the art forthis purpose. Probably the most commonly-used protocol of this type isthe well-known Address Resolution Protocol (ARP), which is described byPlummer in Request for Comments (RFC) 826 of the Internet EngineeringTask Force (IETF), entitled “An Ethernet Address Resolution Protocol”(1982), which is incorporated herein by reference. Given a particularInternet Protocol (IP) destination address, ARP determines the MACaddress to which packets to this IP address should be forwarded.Although ARP relates specifically to IP and Ethernet addressing, other,similar protocols are known in the art for resolving other network layer(Layer 3) and MAC layer (Layer 2) addressing schemes. Therefore,although certain techniques are described hereinbelow with reference toARP, these techniques may be extended in a straightforward manner toother protocols and addressing schemes.

[0054] Mobile station 24 sends an ARP request 64 in order to determinethe MAC address to which it should send data packets destined for acertain IP address. The ARP request is contained in a data frame sentover the WLAN, with a broadcast MAC address, as is known in the art.Therefore, both AP1 and AP2 (and other access points receiving the ARPrequest) forward the request to manager 30, at a forwarding step 66.Manager 30 generates ARP response instructions, at a response generationstep 68. The ARP response to mobile station 64 informs the mobilestation that the MAC address to which data packets to the desired IPaddress should be sent is the MAC address of the selected access point,AP1. Access point AP1 returns the appropriate ARP response 70 to themobile station. All subsequent ARP requests by the mobile station, withrespect to other IP addresses, receive the same response. In thismanner, all entries in the ARP table of each mobile station 24 will bemade to contain the same MAC address—that of the access point that wasselected to serve the particular mobile station.

[0055] Having resolved the required MAC address, mobile station 24 cannow send a data message 72 to this address. The data message comprises apacket, such as an IP packet with the appropriate IP destinationaddress, encapsulated in a MAC frame with the MAC destination address ofAP1. When access point AP1 receives this message, it ascertains that theMAC destination address matches its own assigned address, andimmediately sends an acknowledgment (ACK) 74 back to the mobile station.Other access points ignore message 72, since it is not addressed totheir MAC addresses. AP1 forwards the data message to manager 30 forprocessing, at a forwarding step 76. Manager 30 processes the message ata message processing step 78. Typically, the manager reads the IPdestination address (or other network address) in the packet header inorder to determine whether the data message should be routed or bridgedto another node in system 20, such as another one of mobile stations 24,or should be routed or bridged out of system 20 to network 32.

[0056] In either case, when manager 30 receives a reply to message 72(or other incoming data for mobile station 24), it passes the reply tothe selected access point, AP1, which then transmits a data response 79to the mobile station. The mobile station may send additional datamessages, including further ARP requests, which are handled in themanner described above.

[0057]FIG. 3 is a flow chart that schematically shows a method forhanding over mobile station 24 from one access point to another, inaccordance with an embodiment of the present invention. The method isagain described for simplicity with reference to two access points: AP1and AP2. It is invoked periodically, when mobile station 24 transmits anuplink packet 80 (which may be either a data frame or a management orcontrol frame). It is assumed initially that AP1 has been selected toserve this mobile station. Nevertheless, all access points receiving thepacket measure the received signal strength, at a signal measurement 82.Based on this measurement, each access point transmits a RSSI to manager30.

[0058] Manager 30 compares the RSSI values transmitted by the variousaccess points in order to determine whether a different access pointshould be selected to serve mobile station 24, instead of AP1, at ahandover decision step 84. Typically, if the RSSI value of anotheraccess point, such as AP2, is significantly stronger than that of AP1,manager 30 will decide to make the handover. The change in relativesignal strengths between AP1 and AP2 may be due to movement of themobile station within the service region of system 20, for example, ordue to other changes, such as movement of people or objects in theservice region. Manager 30 may also decide to change the serving accesspoint for a given mobile stations based on other considerations, such asload balancing or network management constraints.

[0059] In order to hand over mobile station 24 from AP1 to AP2, the ARPtable held by the mobile station must be modified to point to the MACaddress of AP2, rather than AP1. For this purpose, manager 30 a spoofedARP response 86, and conveys the ARP response to AP1 for transmission tothe mobile station. (A spoofed ARP response is one that is sentindependently of any ARP request from the mobile station. The structureof ARP is such that the recipient of the spoofed ARP response willtypically update its ARP table accordingly, even though it has made noARP request.) Spoofed ARP response 86 instructs mobile station 24 toreplace all the entries in its ARP table with new entries containing theMAC address of AP2. Typically, manager 30 maintains a mirror of the ARPtable of each mobile station 24, based on the ARP requests received fromthe mobile station, and thus generates the spoofed ARP response withrespect to every one of the entries in the table. Alternatively, manager30 may generate spoofed ARP responses on the fly, whenever the mobilestation sends a packet to AP1 after the handover has taken place. Ineither case, the mobile station receives the ARP response and updatesits ARP table accordingly, at an ARP update step 88.

[0060] Thus, the next time mobile station 24 sends a data message 90,the MAC destination address of the data message will be the MAC addressof AP2. As a result, AP1 will ignore message 90, while AP2 sends ACK 74,forwards the message to manager 30 at step 76, and returns response 79,as described above.

[0061]FIG. 4 is a flow chart that schematically illustrates a method forresponding to an uplink message 100 from mobile station 24, inaccordance with an alternative embodiment of the present invention. Thismethod is intended to alleviate the problem of slow response by accesspoints 22, as mentioned above. After transmitting uplink message 100(such as association request 56 or ARP request 64, for example), mobilestation 24 will generate a timeout 104 if it receives no acknowledgmentwithin the predefined time limit, typically 101 s. The mobile stationwill then automatically retransmit the uplink message, at aretransmission step 106. This cycle of timeout and retransmission may berepeated until the mobile station finally receives a response, or untilthe mobile station decides that its connection to WLAN system 20 hasfailed. Even when such a failure does not occur, the repeatedretransmissions consume radio bandwidth in system 20 that couldotherwise be used for data transmissions.

[0062] Access points AP1 and AP2 forward uplink message 100 to manager30, at a forwarding step 102, as described above. The manager instructsAP1 to serve mobile station 24, at an instruction step 108. Beforeactually responding to the uplink message, manager 30 orders accesspoint AP1 to switch from its usual mobile station emulation (STA) modeto an access point (AP) mode, at a mode switching step 110. As notedabove, in the station-emulation mode, the access point acknowledges onlyuplink packets that are directed to its own assigned MAC address. In APmode, however, the access point acknowledges all uplink messages sent toits assigned BSSID. Therefore, upon receiving the uplink messageretransmitted by mobile station 24 at step 106, AP1 immediately returnsan acknowledgment 112 to the mobile station, as required by the 802.11standard. Alternatively, manager 30 may, at step 110, temporarily switchthe MAC address of AP1 to correspond to the destination MAC address ofuplink message 100, which will likewise cause AP1 to acknowledge theretransmitted uplink message.

[0063] Access point AP1 forwards the retransmitted uplink message tomanager 30, at a forwarding step 114. The manager prepares responseinstructions, at a response generation step 116, as described above.Meanwhile, AP1 returns (either autonomously or under instructions frommanager 30) to its usual mobile station emulation mode, at a moderestoration step 118. Upon receiving the response instructions frommanager 30, AP1 sends a downlink message 120 to mobile station 24. Themobile station may then send a subsequent uplink message 122, and theprocess continues as described above.

[0064] Although the operation of WLAN system 20 is described hereinabovewith reference to the 802.11 family of standards, using certainprotocols and terminology taken from these and other standards, theprinciples of the present invention may similarly be applied in wirelessLANs of other types, based on other standards and communicationprotocols. In addition, these principles may be applied in wirelesspersonal area networks (PANs), as defined by IEEE Standard 802.15,including ultra-wide band (UWB) PANs.

[0065] It will thus be appreciated that the embodiments described aboveare cited by way of example, and that the present invention is notlimited to what has been particularly shown and described hereinabove.Rather, the scope of the present invention includes both combinationsand subcombinations of the various features described hereinabove, aswell as variations and modifications thereof which would occur topersons skilled in the art upon reading the foregoing description andwhich are not disclosed in the prior art.

1. A method for mobile communication, comprising: arranging a pluralityof access points in a wireless local area network (WLAN) to communicateover the air with a mobile station using a common basic service setidentification (BSSID) for all the access points; receiving at one ormore of the access points an uplink signal transmitted over the WLAN bythe mobile station using the common BSSID; sending and receivingmessages over a communication medium linking the access points in orderto select one of the access points to respond to the uplink signal; andtransmitting a response from the selected one of the access points tothe mobile station.
 2. The method according to claim 1, wherein theaccess points are configured to communicate with the mobile station overa common frequency channel shared by all the access points.
 3. Themethod according to claim 2, wherein the access points have respectiveservice areas, and wherein arranging the plurality of the access pointscomprises arranging the access points so that the service areassubstantially overlap.
 4. The method according to claim 1, whereinarranging the plurality of the access points comprises arranging theaccess points to communicate with the mobile station substantially inaccordance with IEEE Standard 802.11.
 5. The method according to claim1, wherein arranging the plurality of the access points comprisesassigning a respective medium access control (MAC) address to each ofthe access points, so that each of the access points ignores uplink datamessages that are not addressed to the respective MAC address.
 6. Themethod according to claim 5, wherein arranging the plurality of theaccess points comprises configuring the access points to emulate mobilestation communications, so that each of the access points acknowledgesthe uplink data messages that are addressed from the mobile station tothe respective MAC address.
 7. The method according to claim 6, whereinsending and receiving the messages comprises reconfiguring the selectedone of the access points temporarily to stop emulating the mobilestation communications, so as to transmit an acknowledgment to amanagement frame transmitted by the mobile station.
 8. The methodaccording to claim 6, wherein sending and receiving the messagescomprises changing the respective MAC address of the selected one of theaccess points temporarily, so as to cause the selected one of the accesspoints to transmit an acknowledgment to a management frame transmittedby the mobile station.
 9. The method according to claim 5, whereintransmitting the response comprises instructing the mobile station totransmit all the uplink data messages to the respective MAC address ofthe selected one of the access points.
 10. The method according to claim9, wherein instructing the mobile station comprises sending an AddressResolution Protocol (ARP) response to the mobile station.
 11. The methodaccording to claim 10, and comprising, subsequently to transmitting theresponse from the selected one of the access points, and responsively toa further uplink signal received from the mobile station, selecting afurther one of the access points to communicate with the mobile station,and sending a spoofed ARP response to the mobile station instructing themobile station to transmit all further uplink data messages to therespective MAC address of the further one of the access points.
 12. Themethod according to claim 1, wherein receiving the uplink signalcomprises measuring a strength of the uplink signal at each of the oneor more of the access points, and wherein sending and receiving themessages comprises indicating in the messages the measured strength ofthe uplink signal, and selecting the one of the access points to respondto the uplink signal responsively to the strength indicated in themessages.
 13. The method according to claim 12, wherein measuring thestrength comprises measuring the strength repeatedly in response tosubsequent uplink signals transmitted by the mobile station, and whereinselecting the one of the access points comprises selecting a differentone of the access points to respond to the subsequent uplink signals,responsively to a change in the measured strength.
 14. The methodaccording to claim 1, wherein sending and receiving the messagescomprises sending the messages from the access points to a manager node,which processes the messages so as to select the one of the accesspoints to respond to the uplink signal, and sending instructions fromthe manager node to the selected one of the access points to transmitthe response.
 15. A method for mobile communication, comprising:arranging a plurality of access points in a wireless local area network(WLAN) to communicate over the air on a common frequency channel with amobile station; receiving at one or more of the access points an uplinksignal transmitted over the WLAN by the mobile station on the commonfrequency channel; conveying messages responsively to the uplink signalfrom the one or more of the access points over a communication mediumlinking the access points to a manager node; processing the messages atthe manager node so as to select one of the access points to respond tothe uplink signal; and transmitting a response from the selected one ofthe access points to the mobile station.
 16. The method according toclaim 15, wherein the access points have respective service areas, andwherein arranging the plurality of the access points comprises arrangingthe access points so that the service areas substantially overlap. 17.The method according to claim 15, wherein arranging the plurality of theaccess points comprises arranging the access points to communicate withthe mobile station substantially in accordance with IEEE Standard802.11.
 18. The method according to claim 15, wherein arranging theplurality of the access points comprises assigning a respective mediumaccess control (MAC) address to each of the access points, so that eachof the access points acknowledges uplink data messages addressed fromthe mobile station to the respective MAC address.
 19. The methodaccording to claim 18, wherein arranging the access points tocommunicate comprises configuring the access points to emulate mobilestation communications, so that each of the access points ignores theuplink data messages that are not addressed to the respective MACaddress.
 20. The method according to claim 19, wherein transmitting theresponse comprises instructing the selected one of the access pointstemporarily to stop emulating the mobile station communications, so asto transmit an acknowledgment to a management frame transmitted by themobile station.
 21. The method according to claim 19, whereintransmitting the response comprises changing the respective MAC addressof the selected one of the access points temporarily, so as to cause theselected one of the access points to transmit an acknowledgment to amanagement frame transmitted by the mobile station.
 22. The methodaccording to claim 18, wherein transmitting the response comprisesinstructing the mobile station to transmit all the uplink data messagesto the respective MAC address of the selected one of the access points.23. The method according to claim 22, wherein instructing the mobilestation comprises sending an Address Resolution Protocol (ARP) responseto the mobile station.
 24. The method according to claim 23, andcomprising, subsequently to transmitting the response from the selectedone of the access points, and responsively to a further uplink signalreceived from the mobile station, selecting a further one of the accesspoints to communicate with the mobile station, and sending a spoofed ARPresponse to the mobile station instructing the mobile station totransmit all further uplink data messages to the respective MAC addressof the further one of the access points.
 25. The method according toclaim 15, wherein receiving the uplink signal comprises measuring astrength of the uplink signal at each of the one or more of the accesspoints, and wherein conveying the messages comprises indicating in themessages the measured strength of the uplink signal, and whereinprocessing the messages comprises selecting the one of the access pointsto respond to the uplink signal responsively to the strength indicatedin the messages.
 26. The method according to claim 25, wherein measuringthe strength comprises measuring the strength repeatedly in response tosubsequent uplink signals transmitted by the mobile station, and whereinselecting the one of the access points comprises selecting a differentone of the access points to respond to the subsequent uplink signals,responsively to a change in the measured strength.
 27. The methodaccording to claim 15, wherein arranging the plurality of the accesspoints comprises assigning all the access points to the same basicservice set (BSS).
 28. The method according to claim 15, wherein themanager node comprises a plurality of management processors.
 29. Themethod according to claim 28, wherein the plurality of managementprocessors comprises a control processor and a packet processor, andwherein processing the messages comprises selecting the one of theaccess points to respond to the uplink signal using the controlprocessor, and further comprises processing uplink data packets receivedby the selected one of the access points using the packet processor. 30.The method according to claim 29, wherein processing the uplink datapackets comprises decrypting the uplink data packets and encryptingdownlink data packets at the packet processor, for transmission by theselected one of the access points.
 31. The method according to claim 28,wherein processing the messages comprises distributing the messages forprocessing among the plurality of the management processors. 32.Apparatus for mobile communication, comprising: a plurality of accesspoints, which are arranged in a wireless local area network (WLAN) tocommunicate over the air on a common frequency channel with a mobilestation using a common basic service set identification (BSSID) for allthe access points, and which are adapted, upon receiving at one or moreof the access points an uplink signal transmitted over the WLAN by themobile station on the common frequency channel, to convey messagesresponsively to the uplink signal from the one or more of the accesspoints over a communication medium linking the access points; and amanager node, linked to the communication medium, which is adapted toprocess the messages so as to select one of the access points to respondto the uplink signal, and to instruct the selected one of the accesspoints to transmit a response to the mobile station.
 33. The apparatusaccording to claim 32, wherein the access points have respective serviceareas, and are arranged so that the service areas substantially overlap.34. The apparatus according to claim 32, wherein the access points areconfigured to communicate with the mobile station substantially inaccordance with IEEE Standard 802.11.
 35. The apparatus according toclaim 32, wherein each of the access points is assigned a respectivemedium access control (MAC) address, so that each of the access pointsignores uplink data messages that are not addressed to the respectiveMAC address.
 36. The apparatus according to claim 35, wherein the accesspoints are configured to emulate mobile station communications, so thateach of the access points acknowledges the uplink data messages that areaddressed from the mobile station to the respective MAC address.
 37. Theapparatus according to claim 36, wherein the manager node is adapted toinstruct the selected one of the access points temporarily to stopemulating the mobile station communications, so as to transmit anacknowledgment to a management frame transmitted by the mobile station.38. The apparatus according to claim 36, wherein the manager node isadapted to temporarily change the respective MAC address of the selectedone of the access points, so as to cause the selected one of the accesspoints to transmit an acknowledgment to a management frame transmittedby the mobile station.
 39. The apparatus according to claim 35, whereinthe selected one of the access points is adapted, under control of themanager node, to instruct the mobile station to transmit all the uplinkdata messages to the respective MAC address of the selected one of theaccess points.
 40. The apparatus according to claim 39, wherein theselected one of the access points is adapted to instruct the mobilestation to transmit all the uplink data messages to a given MAC addressby sending an Address Resolution Protocol (ARP) response to the mobilestation.
 41. The apparatus according to claim 40, wherein the managernode is adapted, subsequently to transmission of the response to themobile station, and responsively to a further uplink signal receivedfrom the mobile station, to select a further one of the access points tocommunicate with the mobile station, and to cause the selected one ofthe access points to transmit a spoofed ARP response to the mobilestation instructing the mobile station to transmit all further uplinkdata messages to the respective MAC address of the further one of theaccess points.
 42. The apparatus according to claim 32, wherein theaccess points are adapted to measure a strength of the uplink signalreceived at each of the one or more of the access points, and toindicate in the messages conveyed over the communication medium themeasured strength of the uplink signal, and wherein the manager node isadapted to select the one of the access points to respond to the uplinksignal responsively to the strength indicated in the messages.
 43. Theapparatus according to claim 42, wherein the access points are adaptedto measure the strength repeatedly in response to subsequent uplinksignals transmitted by the mobile station, and wherein the manager nodeis adapted to select a different one of the access points to respond tothe subsequent uplink signals, responsively to a change in the measuredstrength.
 44. The apparatus according to claim 32, wherein the managernode comprises a plurality of management processors.
 45. The apparatusaccording to claim 44, wherein the plurality of management processorscomprises: a control processor, which is adapted to select the one ofthe access points to respond to the uplink signal; and a packetprocessor, which is adapted to process uplink data packets received bythe selected one of the access points.
 46. The apparatus according toclaim 45, wherein the packet processor is adapted to decrypt the uplinkdata packets and to encrypt downlink data packets for transmission bythe selected one of the access points.
 47. The apparatus according toclaim 44, wherein the management processors are adapted to process themessages by distributing the messages among the management processors.